Method of colour electrophotography

ABSTRACT

A method of electrophotography comprising a first step of applying a first electric field across a photosensitive element including a transparent electrode, a photosensitive photoconductive layer exhibiting persisting internal polarization and a highly insulative layer to deposit a charge of one polarity on the surface of the highly insulative layer, a second step of applying a second electric field across the photosensitive element to deposit a charge of the opposite polarity on the surface of the highly insulative layer concurrently with the projection of a light image upon the photosensitive photoconductive layer through the transparent electrode layer whereby to form a latent image corresponding to the light image on the surface of the highly insulative layer and a third step of developing the latent image with a toner to form a powder image, said first, second and third steps being repeated successively with light images of different colors, and toners of different colors while the photosensitive element is held stationary whereby to form a plurality of powder images of different colors which are superposed upon each other.

United States Patent [1 1 Kinoshita et al.

[ METHOD OF COLOUR ELECTROPl-IOTOGRAPHY [75] Inventors: Koichi Kinoshita, Narashino;

Masanori Watanabe, Tokyo-to, both of Japan [73] Assignee: Katsuragawa Denki Kabushiki Kaisha, Tokyo-to, Japan 22 Filed: Dec. 3, 1970 21 Appl. No.2 94,794

[30] Foreign Application Priority Data Dec. 17, 1969 Japan 44-101500 [52] US. Cl. 96/1.2, 96/1.4, 1l7/l7.5,

' 355/4 [51] Int. Cl... 603g 13/22 [58] Field of Search 96/1, 1.2, l R, 1.4; 117/175; 355/4 [56] References Cited UNITED STATES PATENTS 3,615,383 10/1971 lnouc et al..,. 96/1 3,655,369 4/1972 Kinoshita..... 96/1 3,178,281 4/1965 Jarius 96/1 3,043,686 7/1962 Bickmore.....,, 96/] 3,438,706 4/1969 Tanaka et a1. 355/11 3,457,070 7/1969 Watanabe et al 96/ 1.4 3,376,133 4/1968 Roteman 96/1 .2

I451 Apr. 30,.1974

Primary Examiner-Charles E. Van Horn Attorney, Agent, or FirmBosworth,- Sessions, Herrstrom & Cain ABSTRACT layer todeposit a charge of one polarity on the surface of the highly insulative layer, a second step of applying a second electric field across the photosensitive element to deposit a charge of the opposite polarity on the surface of the highly insulative layer concurrently with the projection of a light image upon the photosensitive photoconductive layer through the transparent electrode layer whereby to form a latent image corresponding to the light image on the surface of the highly insulative layer and a third step of developing the latent image with a toner to form a powder image, said first, second and third steps being repeated successively with light images of different colors. and toners of different colors while the photosensitive element is held stationary whereby to form a plurality of powder images of different colors which are superposed upon each other.

7 Claims, 3 Drawing Figures PATENTEDAFR 30 IBM FIG.

FIG. 2

FIG. 3

' INVENTOR5 .KOlCH/ KINOSH/TA NASA/V0.1?! WAT/4 NABE ATTORNEYS BACKGROUND OF THE INVENTION This invention relates to color electrophotography. While there have been proposed a number of color electrophotography machines they are generally complicated in construction and hence expensive and difficult to handle. Moreover, they can not reproduce intermediate colors or tones at high fidelities, have low resolution or produce images which are out of register.

According to one prior method, just like a color printing machine, a plurality of electrophotographic machine units are arranged side by side and a photosensitive element is successively passed through these machine units to successively form powder images of different colors. In this method, it is difficult to exactly register a plurality of powder images. In addition, the apparatus required is bulky and expensive.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a novel method of color electrophotography capable of reproducing a colored original with high resolution and fidelity.

Another object of this invention is to form color prints without the risk of being out of register with a single electrophotographic .machi'ne unit.

A further object of this invention is to reproduce intermediate colors or tones at high fidelities without the necessity of utilizing a screen.

A still further object of this invention is to readily produce latent images or color printed copies of relatively high contrast although color filters are used.

According to this invention, there is provided a method of color electrophotography comprising a first step of applying a first electric field across a photosensitive element including a transparent electrode layer, a photosensitive photoconductive layer exhibiting persistent internal polarization and a highly insulative layer to deposit a charge of one polarity on the surface of the highly insulative layer; a second step of applying a second field across the photosensitive element to deposit a charge of the opposite polarity on the surface of the highly insulative layer concurrently with the projection of a light image of one color upon the photosensitive photoconductive layer through the transparent electrode layer to form a latent image corresponding to the light image on the surface of the highly insulative layer; a third step of developing the latent image with a toner of said one color to form a first powder image of said one color; and a fourth step of repeating successively said first, second and third steps with a light image of another color and a toner of said another color while the photosensitive element is held stationary whereby to form a second powder image of said another color superposed on the first powder image.

As is well known in the art, although the first and second fields may be applied by means of a contact electrode removably placed on the highly insulative layer it is advantageous to use a well known corona discharge unit.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a diagrammatic representation, partly in section, of an electrophotographic apparatus suitable for carrying out the method of this invention, and

FIGS. 2 and 3 are sectional views showing modified photosensitive elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrophotographic apparatus shown in FIG. I employs a photosensitive element 5 which comprises a transparent substrate 1 such as a glass plate, a transparent electrode 2 on one side of substrate 1, a photosensitive photoconductive layer 3 having a plurality of charge trapping levels to exhibit persistent internal polarization and a highly insulative layer 4. The layers are superposed on each other in the order mentioned and are bonded together into an integral unit. The photosensitive element 5 is supported at a definite position or to be movable in the lateral direction with its substrate I faced upwardly. A corona discharge unit 6 is disposed beneath photosensitive element 5 to deposit a charge of any desired polarity on the entire surface of the highly insulative layer 4.-Corona discharge unit 6 is adjustable in the vertical direction or toward and away from the photosensitive element. A charging device including a source of uniform light 8 and a second corona discharge unit 7 constructed to freely transmit the light from source 8 is provided to be movable across the surface of the highly insulative layer 4 to scan the same when the first corona discharge unit 6 is lowered. A plurality of developing units 9a and 9b containing toners of different colors are positioned beneath and on the opposite sides of the photosensitive element. One of them 9a is movable in the vertical direction toward the surface of highly insulative layer 4 and along the surface of the photosensitive element whereas the other 9b is positioned close to the surface of the highly insulative layer 4 to develop the latent image formed thereon when the photosensitive element 5 is moved to the right. An optical lens 12 and sources of light 11 for illuminating an original 10 are disposed above the photosensitive element to project the light image of the original upon photosensitive layer 3 through transparent substrate 1 and transparent electrode 2. A plurality of optical color filters 13a and 13b transmitting light rays of different colors are disposed beneath lens 12 such that any one of them is selectable to cooperate with lens 12. If desired, a light shield 14 may be provided to enclose lens 12, filters 13a and 13b and the upper surface of the photosensitive element 5 to intercept external light at the time of image projection. An AC corona discharge unit 15 and a cleaner 16 are disposed to remove residual charge and toner when the photosensitive element 5 is moved to the left to prepare it for the next cycle. The second corona discharge unit 7 and light source 8 may be omitted when a transparent counter electrode 17 is provided for the first corona discharge unit 6 and light sources 18 are provided beneath it as shown by dotted lines.

A powder image is formed on the surface of the highly insulative layer 4 in the following manner. In the first step, while the photosensitive element 5 is held stationary in the position shown in FIG. I the first corona discharge unit 6 is operated to deposit a charge on the surface of the highly insulative layer 4 of a polarity opposite to the polarity of the majority carriers of the photoconductive material comprising the photosensitive photoconductive layer 3 whereby the majority carriers are trapped in the trapping levels at or near the interface between photoconductive layer 3 and highly insulative layer 4 thus establishing presistent internal polarization.

In the second step, corona discharge unit 6 is operated to deposit a charge of the opposite polarity; that is, a charge of the same polarity as the majority carriers of the photoconductive material on the surface of the highly insulative layer. At the same time the light image of the selected color of the original is projected upon the photosensitive photoconductive layer 3 through lens 12, one of the filters, 13a for example, and transparent electrode 2 so as to preserve the charge trapped in the photoconductive layer 3 during the first step at portions corresponding to the dark portions of the light image and to promptly release such trapped charge at portions corresponding to the bright portions of the light image whereby an electrostatic latent image is formed on the surface of the highly insulative layer 4 corresponding to the light image of the particular color.

In the third step, corona discharge unit 6 is lowered to a position indicated by dot and dash lines and developing unit 9a containing a toner of the same color as that of the color image projected in the second step is raised to a position close to the photosensitive element and is then moved to the right as shown by the arrow to develop the latent image formed on the surface of highly insulative layer 4. Such development can be carried out either in the dark or under room light. Without.

fixing or transfer printing the powder image thus formed, the fourth step is carried out. In this step, another latent image of different color is formed by the steps identical to the first and second steps except that another color filter 13b is used. The latent image of the different color image, which is precisely superposed on the previously formed latent image, is developed by using another developing unit 9b containing'a toner of different color. In this manner, by using a number of filters of different colors and by repeating the above described first to third steps, a powder image of any desired color can be formed which is transfer printed onto a recording paper and is then fixed in a manner well known inv the art. Intermediate colors can be reproduced at high fidelities because toners of different colors are suitably admixed at the time of fixing.

It is particularly to be noted that the powder image formed by the first image forming procedure is not destroyed or degraded by the second image forming and developing procedure. It is presumed that this is because the first powder image is held on the highly insulative layer by the electrostatic attractive force.

The charge may be applied in the dark during the first step. However, dependent upon the characteristics of the photosensitive photoconductive layer 3 this charge may be applied while uniform light is irradiated over the entire surface of the photosensitive element. Thus, for example, during the first step, the first corona discharge unit 6 is lowered to the position shown by dot and dash lines and then the light transmitting second corona discharge unit 7 and associated light source 8 and moved to the right along the lower surface of the photosensitive element to scan the same. Alternatively, where the first corona discharge unit 6 is provided with transparent counter electrode 17, lamps 18 are lighted concurrently with the operation of corona discharge unit 6. In this case, it is advantageous to reciprocate corona discharge unit 6 in the longitudinal direction (to the left and right) over a small distance in order to eliminate non-uniform charging caused by the shadow of the discharge wires.

After transfer printing of the color powder image it is necessary to remove residual charge and toners from the photosensitive element. To this end, the photosensitive element 5 is moved to the left as shown by dot and dash lines to erase the residual latent image with the AC corona discharge unit 15 and to remove remaining toners from the surface of the highly insulative layer 4 by means of the cleaner 16. Thereafter, the element 5 is ready for use in the next cycle of image formation.

The photosensitive element utilized in this invention may take various other forms. For example, as shown in FIG. 2, another highly insulative and transparent layer 4' may be interposed between transparent electrode 2 and photosensitive photoconductive layer 3 and integrally bonded to both layer3 and electrode 2. Alternatively, as shown in FIG. 3, a charge trapping layer 3' having a large light absorption coefficient and which can produce many free charge carriers when irradiated with light may be provided between highly insulative layer 4 (or electrode 2 when layer 4' is not used) and photosensitive photoconductive layer 3. In

this construction, the photoconductive layer 3 is composed of material that permits free charge carriers injected from charge trapping layer 3' to migrate over long distances and the material forms only a small number of free charge carriers when it is excited thermally, thus providing a photosensitive element of high resolution and high photosensitivety. The charge trapping layer 3 may be formed by the concurrent vapour deposition of Se and a SeTe alloy containing a relatively large quantity of Te, whereas the photosensitive photoconductive layer 3 may be formed by the vapour deposition of a SeTe alloy containing a relatively small quantity of Te. When these two layers are formed by consecutive vapour deposition steps, a well defined interface will not be formed therebetween, but the composition will vary gradually from one layer to the other. Advantageously, the thickness of both highly insulative layers 4 and 4 may be a few microns but the layer 4 may be about 1 micron thick. Further, in the embodiment shown in FIG. 3 the highly insulative layer 4' may be omitted.

Developing units 9a and 9b may be of the powder clouding type wherein the powdery toner is sprayed onto the surface of the highly insulative layer 4 or a cascade type wherein the powdery toner is sprinkled over the highly insulative layer. When using a developing unit of the magnetic brush type it is necessary to take care not to mechanically scratch the surface of the highly insulative layer. Summarizing the above this invention has the following advantages.

1. As the light image is projected upon the photosensitive photoconductive layer through the transparent substrate and the transparent electrode to form the latent image on the surface of the highly insulative layer located on the side of element 5 opposite to the substrate, the projected light image is not intercepted by the powder image even though a number of latent image forming steps and a number of developing steps by toners are performed alternately. For this reason, it is possible to precisely overlap a plurality of images of different colors 'thus reproducing intermediate colors at high fidelities. Moreover, as it is not necessary to use a screen to decompose the image into a number of discrete elements it is possible to preserve a high degree of resolution.

2. As the high contrast latent image is formed on the surface of the highly insulative layer by partially or selectively releasing the charge that has been trapped in the photoconductive layer during the first step by the projection of the light image and application of the electric field during the second step it is possible to readily produce latent images of relatively high contrast irrespective of the decrease in the light quantity caused by the use of color filters and transparent electro e.

3. As two or more image forming steps and developing steps are successively carried out while the photosensitive element is held at a fixed position it is possible to obtain clear color prints which are not unduly out of register.

The invention is not limited to the particular embodiment illustrated. For example, the original sometimes contains black portions in addition to colored patterns. In such a case, an additional step is used wherein the light image is projected without utilizing an optical filter and the latent image is developed with a black toner.

We claim:

l. A method of color electrophotography comprising a first step of applying a first electric field across a photosensitive element including a transparent electrode layer, a photoconductive layer exhibiting persistent internal polarization and a highly insulative layer to deposit a charge of one polarity on the surface of said highly insulative layer; said layers being superposed on each other in the order mentioned and being bonded together into an integral unit; a second step of applying a second electric field across said photosensitive element to deposit a charge of the opposite polarity on the surface of said highly insulative layer concurrently with the projection of a light image of one color upon said photoconductive layer from the side of said element opposite the highly insulative layer and through said transparent electrode layer to form a latent image corresponding to said light image on the surface of said highly insulative layer; a third step of developing said latent image with a toner of said one color to form a first powder image of said one color; and a fourth step of repeating said first, second and third steps with a light image of another color and a toner of said another color, at least said first and second steps being repeated without moving said photosensitive element from the location where at least the said second and third steps were first carried out whereby to form a second powder image of said another color superposed on said first powder image.

2. The method according to claim 1 wherein said photosensitive element is illuminated with uniform light from the side on which said highly insulative layer is disposed substantially concurrently with the application of said first field.

3. The method according to claim I wherein said powder images are transfer printed onto a recording paper.

4. The method according to claim 1 wherein optical filters of different colors are selectively used to project said light image of different colors.

5. The method according to claim 1 wherein in addition to said colored powder images a black powder image is also formed in superposed relation to said color images.

6. The method according to claim 1 wherein said first and second fields are applied across a photosensitive element comprising a transparent electrode layer, a first highly insulative and transparent layer, a photosensitive photoconductive layer exhibiting persistent internal polarization and a second highly insulative layer, said layers being superposed in the order mentioned and being bonded together into an integral unit and wherein said powder images are formed on the surface of said second highly insulative layer.

7. The method according to claim 1 wherein said first and second fields are applied across a photosensitive element comprising a transparent electrode layer, a charge trapping layer comprising a vapor deposited layer composed of selenium and an alloy of selenium and tellurium having a plurality of charge trapping levels, a photoconductive layer and a highly insulative layer, said layers being superposed on each other and being bonded together to form an integral unit.

UNI'IED STATILS IVA'IRN'I OFFICE CERTIFICATE OF CORR ECTlON HIM-M v.3L8 Li SI! .Ae ziliq1.321.-mom

qhoichi KINOSHITA and Masanori' WATANABE I "U; is certi fied that: error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The title should be corrected to read -Method of Color Electrophotography. I

Column 3 line 1, correct the spelling of -persistent.

Column 3, line 60, before "moved" change "and" to are-.

Signed and sealed this 10th day of September 1974.

(SEAL) Attestz MCCOY M. GIBSON, JR. C. MARSHALL .DANN Attesting Officer Commissioner of Patents UNl'lED STATES PA'IEN'I OFFICE (I EH1! FICATE OF COR l'HHI'llON Koichi KINOSHITA and Masanori WATANABE l nventor it is ce tified that error appears in the above-identified patent and thnt said Letters Patent are hereby corrected as shown below:

The title should be corrected to read --Method of Color Electrophotography.

Column 3, line 1, correct the spelling of -persistent--.

Column 3, line 60, before "moved" change "and" to -are.

Signed and sealed this 10th day of September 1974.

(SEAL) Attest:

C. MARSHALL DANN McCOY M. GIBSON, JR.

Commissioner of Patents Attesting Officer 

2. The method according to claim 1 wherein said photosensitive element is illuminated with uniform light from the side on which said highly insulative layer is disposed substantially concurrently with the application of said first field.
 3. The method according to claim 1 wherein said powder images are transfer printed onto a recording paper.
 4. The method according to claim 1 wherein optical filters of different colors are selectively used to project said light image of different colors.
 5. The method according to claim 1 wherein in addition to said colored powder images a black powder image is also formed in superposed relation to said color images.
 6. The method according to claim 1 wherein said first and second fields are applied across a photosensitive element comprising a transparent electrode layer, a first highly insulative and transparent layer, a photosensitive photoconductive layer exhibiting persistent internal polarization and a second highly insulative layer, said layers being superposed in the order mentioned and being bonded together into an integral unit and wherein said powder images are formed on the surface of said second highly insulative layer.
 7. The method according to claim 1 wherein said first and second fields are applied across a photosensitive element comprising a transparent electrode layer, a charge trapping layer comprising a vapor deposited layer composed of selenium and an alloy of selenium and tellurium having a plurality of charge trapping levels, a photoconductive layer and a highly insulative layer, said layers bEing superposed on each other and being bonded together to form an integral unit. 